Microwave antennas are widely used in communications, radioastronomy, radiotelemetry, radars, etc. It has also been widely proposed and experimented to use electromagnetic waves for energy transmission between two separated locations. There is a need for a cost-effective means for the reception and conversion of electromagnetic power to direct current power more suitable for moving platforms on which the reception/conversion system is located. A rectifying antenna is customarily called a rectenna and includes antenna elements and rectifiers directly connected to them to produce a direct current output. An exemplary application of the rectenna in which this need arises is the provisioning of 30 KW or more of propulsive and communications payload power for lightweight electrically-powered aircraft. In operation, such aircraft would circle over fixed ground antenna systems, transmitting power in the 2.4 to 2.5 GHz microwave ISM band, for continuous periods of weeks or months at a time and relay communication signals between separated locations.
Of course, there are many other applications in which the supply of energy to a remotely located station is desired in the form of electromagnetic waves, thus eliminating the needs of physical connections, e.g. wires, pipes, and permitting the station to be movable. It is also advantageous to provide antennas which can perform equally well for microwaves of various polarizations.
Various microstrip array antennas have been proposed for microwave uses. U.S. Pat. No. 4,464,663 to Larezari et al (Aug. 7, 1984) describes a dual polarized microstrip antenna. The antenna comprises a pair of spaced apart resonant microstrip radiators and specifically designed x and y feedlines which achieve respective polarizations while minimizing undesirable rf coupling between x and y input/output ports. While it is an important consideration to achieve good polarization isolation in the fields such as communications, radars, etc., power reception by microwave antennas requires optimum sensitivity to signals regardless of the polarization.
U.S. Pat. No. Re: 29,911 to Munson (Feb. 13, 1979) teaches a high gain phased array antenna which is, in his preferred embodiment, made by the printed circuit board technique. While described as possible to radiate linearly and/or circularly polarized radiation, the feedline designs indicate that the antenna is not equally sensitive to x and y polarizations.
U.S. Pat. No. 4,943,811 July 24, 1990 (Alden et al) describes a dual polarization power reception and conversion system. This device consists of two orthogonal arrays of linearly-polarized thin film rectennas of specific format and element spacings. This antenna has proven to be highly efficient and to have a wide range of angles of reception. However, it has certain drawbacks in its manufacture, mechanical assembly and power handling capability. Each of the two rectenna foreplanes is manufactured by etching of both sides of the conductor-clad dielectric sheet from which it is made, with close registration required between back and front circuit elements. These four etching steps become increasingly problematic and costly as the system frequency increases. In addition, the system thickness required is approximately .lambda..sub.o /4 or more, where .lambda..sub.o is the wavelength of the electromagnetic energy in free space. At lower microwave frequencies this can result in a system thickness preventing true conformal application. That is, the rectenna structure has to be integrated mechanically with both the skin and support structure of the moving platform, with only approved dielectric allowed between foreplanes and reflector. The mechanical assembly is also complicated by the requirement of insulation between antenna foreplanes. Thirdly, the power handling capability of this prior art system is limited to one rectification unit for each polarization with power dissipation limited to radiative and convective cooling of the exposed foreplanes only.
U.S. Pat. No. 4,079,268 to Fletcher et al (Mar. 14, 1978) describes an alternative power conversion system. This design eliminates the manufacturing, installation and power handling problems discussed above but is only applicable to a circularly polarized transmission system. Such a system, requiring correct phasing of orthogonal polarizations, may be considerably more complex and costly than the linear or dual transmitter system and is also susceptible to performance degradation due to depolarization.